The use of silicon (Si) to manufacture integrated circuits (ICs) is a well-developed technology. Since early years of IC development, silicon has dominated the field of electronics to become the most widely used material to fabricate and integrate various electronic devices, in particular solid-state transistors such as bipolar and metal oxide semiconductor (MOS) transistors. Consequently, silicon technology has significantly advanced in comparison to other alternative technologies. As a result, Si-based ICs have a number of advantages over other technologies. For example, Si-based ICs can be manufactured with relative ease using established semiconductor processes. In addition, an extremely high density of electronic devices can be fabricated on Si-based ICs.
The preference of Si-based ICs makes it desirable to fabricate photosensitive devices in this material, such as photodiodes and phototransistors, so that the photosensitive devices and the supporting circuit to drive the devices can be integrated. However, due to the bandgap of Si, photosensitive devices fabricated on Si do not operate well for detecting light having wavelengths longer than approximately 850 nm and are somewhat less efficient due to the indirect nature of bandgap. Unfortunately, for longer distances, fiber optic devices favor wavelengths longer than 850 nm, such as 980 nm or 1300 nm, because attenuation and dispersion effects are lower at these longer wavelengths. In addition, the longer wavelengths allow single mode operation.
Si-based photodiodes that are sensitive to light having wavelength greater than 850 nm are described in various U.S. patents. These Si-based photodiodes devices use germanium (Ge) to absorb longer wavelength photons, e.g. 1300 nm photons. Unlike Si, which has a bandgap of 1.1 eV, Ge has a bandgap of 0.67 eV. Thus, Ge can be used to more efficiently absorb 1300 nm photons. As an example, U.S. Pat. No. 6,075,253 issued to Sugiyama et al. describes a semiconductor photodetector that includes a photo-absorption Ge monocrystal layer sandwiched between an n-type Ge layer and a p-type Ge layer within in a recess formed on a Si substrate. The described semiconductor photodetector further includes a p-type layer on the p-type Ge layer within the recess. The semiconductor photodetector of Sugiyama et al. is described as being sensitive to light having wavelength of 1000 nm or greater.
A concern with conventional Si-based Ge photodiodes is that, when used in optical detectors, their sensitivity to longer wavelength light may not be sufficient for certain applications. In addition, conventional Si-based Ge photodiodes may require a more complex fabrication process than other types of photodiodes, which translates into increased manufacturing cost.
In view of these concerns, there is a need for an optical detector and method for detecting incident light that utilizes a Si-based Ge photosensitive device, which can be fabricated using a standard SiGe fabrication process, and has high sensitivity for detecting longer wavelength light, such as 980 nm or 1300 nm light.